Method of making prestressed structural member



May 26, 1959 c. DQBELL BETH 0D OF MAKING PRESTRESSED STRUCTURAL MEMBEROriginal Filed April 3, 1950,

2 Sheets-Sheet 1 INVENTOR ATTORNEY;

y 26, 9 c. DOBELL 2,887,762

METHOD OF MAKING PRESTRESSED STRUCTURAL MEMBER Origihal Filed April :5,1950 I 2 Sheets-Sheet 2 hya.

INVENTOR METHOD OF MAKING PRESD STRUCTURAL MEMBER @urzou' Dobell, NewYork, N.Y., assignor to The Preload tCompany, Inc., New York, N.Y., acorporation of Delaware tlllri application April 3, 1950, Serial No.153,544,.

Divided and this application November 15, 1954, Serial No. 468,838

1 Claim. (Cl. 29-155) This invention relates to a structural metalmember and to a method for making a structural metal member of improvedstrength.

Prior to this invention there existed in this art the problem ofproducing a structural metal member which could economically support-agreater load without materially increasing the volume of metal employed.Heretofore, attempts were made to meet this problem by resorting toparticular materials in the construction of structural metal members.These attempts have manifested limited commercial success, the increasein cost more than offsetting the gain in structural strength.

Further activity was exhibited in the prior art in the form of securingmetal bars on rods and turnbuckles to existing structural metal membersalready under load. Thesedevices are in the nature of reinforcements andare not suitable for use in new work.

It is an object of this invention to provide a simple and expedientmethod of making a structural metal memher which overcomes thedeficiencies of the prior art.

It is another object of this invention to provide an improved structuralmetal member'which can be more economically produced than was heretoforeavailable.

It is another object of this. invention to provide a structural metalmember which contains less metal with respect to load carrying capacitythan has heretofore been available.

Other objects and advantages of the present invention will becomeapparent from a study of the following specification when considered inconjunction with the drawings, in which:

Figure 1 is a perspective view of the structural metal member completelyassembled;

Figure 2 is a fragmentary vertical section of the load carrying flange;

Figure 3 is a fragmentary vertical section of the load carrying flangeshowing the tensioning elements Figure 4 is similar to Figure 3 andshows the load carrying flange after the rolling operation;

Figure 5 is a bottom view of a load carrying flange showing theprestressing element tensioned and secured;

Figure 6 is an end view of the structural metal member showing therollers in place;

Figure 7 is a vertical section of a structural metal member showing amodified. embodiment of the invention; and

Figure 8 is a side view of a conventional bridge truss showing themodified embodiment of Figure 7 in use.

Before discussing the invention in detail in conjunction with thedrawings it is desirable to first present generally the nature and scopeof this invention.

The present invention overcomes the inherent defects of the prior art byprestressing the metal members prior to their being utilized as loadcarrying members. The prestressing is accomplished by the inclusion ofprestressing elements such as tensioned wires in the tension surface ofthe structural steel member.

This invention broadly contemplates the use of prestressing means with astructural metal member to greatly improve its strength and loadcarrying capacity. By the method of this invention tensioned highstrength wire or other suitable prestressing means is embedded in theteusion surface of a structural steel member. This pro" duces tensionalforces in the compression surface hereafter called the load bearingsurface. It also produces compressional forces in the tension surfacehereafter called the load carrying surface. These forces exist While thestructural metal'member is in a noload condition.

Upon subjecting the member to a load, the member itself will notbra-stressed until the forces brought about by the prestressing areneutralized. Beyond this new tralization point the load will stress boththe member and. the prestressing means in proportion to their strength,weight, and other physical properties.

As an example of this invention, a structural metal member with anallowable structural code tensile stress of 20,000 p.s.i. may be putunder an initial compression in the tension or load carrying surface of20,000 p.s.i. This is accomplished by the inclusion of high strengthsteel wire tensioned to 120,000 p.s.i. in the load carrying surface. Onsubjecting the member to a. live load, the load carrying flange maychange from 20,000 p.s.i. compression to 20,000 p.s.i. tension, anoverall change of 40,000 p.s.i'. The stress in the wire would changecorrespondingly from 120,000 p.s.i. tension to 160,000 p.s.i. tension.Such a structural metal member would be twice as strong as a member notmade by the method of this invention.

The structural metal member of this invention may be made of any of thematerials commonly used for this purpose, such assteel or the like. Thisinvention, however, has particular applicability with structural memberswhich are made of light metals such as alloys of aluminum, magnesium,and similar materials. llt is known that structural members constructedof these light metals possess the disadvantage of deflecting much morethan steel members for the same cross-section and load due to theirlower modulus of elasticity. Although the allowable code stresses forlight metal are approxi' equal, if not greater than those for steel, theprob of deflection is important from a design standpoint. However, whena light metal member is'prestressed with steel wires in accordance withthis-invention, the deflection disadvantage can be eliminated to a majordegree. This is apparent when it is considered that there is an equalstretch of the steel wires and light metal in the tension surface of theprestressed member. However, for the same stretch, there is not an equalchange of loading in the light metal and the steel wires. The steel wirmhave a greater modulus of elasticity and, therefore, assume the load toa degree proportional to the ratio of elasticity moduli of the steelwires and the light metal respectively.

In addition to the foregoing, the manufacturing processes for lightmetal members are quite flexible. In view of the properties of thematerials involved, it is frequcntly possible to extrude light metalshapes. This being the case, slots or holes in the tension surface forreceiving the prestressing elements can be included in the extrusionprocess thereby eliminating the necessity of treating-the member'once itis fabricated. Also prestressing elements such as wires can be includedin the holes or slots during the extrusion process, these wires being ina tensiouedconditiou due to the extrusion pressure.

Referring now to the drawings, Figure 1 shows the completed structuralmetal member 20 in perspective. The member consists generally of anupper load bearing flange 21, a lower load carrying or tensioning flange22, and an intermediate body portion 23 which joins the two flangestogether. The load bearing flange 21 is provided with an upper surface25 upon which a load is placed.

asavgrea The lower load carrying flange 22 is adapted to receive andengagingly embed tensioning elements 24 in the manner describedhereinafter.

Figures 2, 3, 4, and 6 show the method by which the improved structuralmetal member is made.

The first step is to form or cut by suitable means slots 26 extendinglengthwise in the lower surface 28 of the load carrying flange 22.Although the slots 26 are shown as rectangular in section and extendingparallel to the long axis of the I beam, they may be of any shape andthey may run in any direction which will produce a major force componentparallel to the longitudinal axis of the member 20.

When the formation of the slots 26 is complete, prestressing elements 24are placed in them. The prestressing elements are of a special characterand should be capable of being tensioned to 120,000 p.s.i. or greater,i.e., high strength wire or other equally appropriate material. Theprestressing elements 24 while retained in the slots 26 are tensioned tothe proper degree by conventional means (not shown). As illustrated byFigure 5, a single piece of high strength wire 24 is placed in the slots26 by laying it back and forth in the slots 26, looping it about theend30 of the load carrying flange 22 in the manner indicated at 32. Thehigh strength wire 24 is then tensioned by any conventional means (notshown) and the ends 33 are secured by means of a clamp It is to beunderstood, however, that while only a single length of high strengthwire is shown, it is nevertheless within the scope of this invention toemploy a prestressing element for each slot or group of slots and tomaintain the prestressing element or elements in tensioned fashion byclamps or other desirable means.

After the prestressing element 24 has been tensioned and secured byclamp 31, the structural metal member is rolled in the manner indicatedin Figure 6. During this operation roller 34 is brought to bear on thelower surface 28 of the load carrying flange 22, and rollers 35 and 36are provided to support the load carrying flange 22 of the structuralmetal member. In the course of this operation the material of the lowerflange will be forced around the tensioned prestressing element 24 tohold it securely, and the slots 26 will be closed as indicated at 29 inFigure 3.

Although the tensioned prestressing element 24 may be untreated, it ispreferred that the element 24 be first coated with an abrasive or othermaterial such as carborundum as indicated at 27 of Figure 3. The element24 is coated so that it will be more securely embedded in the loadcarrying flange 22 at the conclusion of the rolling operation.

The clamp 31 may now be released and the structural metal member 20treated by a grinding method to remove the loops 32 which jut out overthe ends 30 of the load carrying flange 22.

In the modified embodiment shown in Figures 7 and 8 the structural metalmember 20 is provided with prestressing elements 24, which have beencoated with an abrasive as shown at 27, in the load carrying flange 22,the intermediate body portion 23, and the load bearing flange 21. Themodification is used in a conventional bridge truss and forms the lowergirder. When employed in such a structure the modified member acts as atension member with the prestressing allowing the member to take twicethe load or greater. This is in view of the fact that members subject topure tension and not subject to bending moments can be made stronger ifthey are first put in compression by prestressing. In this manner abridge truss is developed which is capable of sup- 4 porting a greaterload per unit volume of steel or other material than has previously beenavailable.

While the invention has been described in conjunc on with an I beam, itis of course understood that c \er types of beams or girders may beemployed. These other types include L beams, T beams, built-up beams,box girders and the like. In the case of box girders, which arefrequently fabricated from fiat strips by welding or riveting,prestressed flat bars would be required in the fabrication for thetension face of the girder. These flat bars appearing in the tensionface of the box girder would probably have uniformly distributedprcstressing. However, the flat bars acting as webs and appearing on thesides of the box girder would probably require nonuniform prestressingwith greater concentration on the tension half of the web. This wouldalso apply to any other built-up beam section.

This invention is also applicable to the prestressing of columns andpiles to counteract certain loading, especially where such loadings takeplace eccentrically. The prestressing in constructions of this nature isarranged to present tensional forces at points where compressionalforces will exist due to the loading of the pile or column. Theprestressing will also present compressional forces at points wheretensional forces are set up in the pile or column due to the loadingthereof. In this manner piles and columns can be constructed which willbe adapted to support greater loads and be of improved strength.

What has been described heretofore is a specific embodirnent of thepresent invention. Other and further embodiments obvious to one skilledin the art from this description are within the contemplation and spiritof this invention.

This application is a division of my application Serial No. 153,544,filed April 3, 1950, now abandoned.

I claim:

A method of reinforcing an elongated metal structural member having alower flange and an upper flange connected by a web that comprisesforming a plurality of slots in the surface of the lower flange, saidslots extending along the longitudinal axis of the flange, positioningan elongated high tensile strength metal reinf ing element in each slot,tensioning said reinforcing ment and pressing the lower flange to closesaid slots 4 said reinforcing element while it is under tension to causethe sides of the slots to frictionally engage said element to maintainthe tension thereon and place the lower flange in compression,therebyplacing the upper flange in tension.

References Cited in the file of this patent UNITED STATES PATENTS1,148,223 Feeser July 27, 1915 2,039,398 Dye May 5, 1936 2,244,109 KleinJune 9, 1941 2,375,068 Bennett May 1, 1945 2,463,580 Warshyk May 10,1946 2,607,109 Reynolds Aug. 19, 1952 2,609,586 Parry Sept. 9, 19522,627,649 Matthyse Feb. 10, 1953 FOREIGN PATENTS 5,537 Great BritainFeb. 1, 1890 556,572 Great Britain Oct. 11, 1943 OTHER REFERENCES Pages178182 of the Railway Gazette, August 17, 1951.

